spidermonkey: Fix building 31.5 on GCC 5.

pkgs/development/interpreters/spidermonkey/31.5.nix

···
   postUnpack = "sourceRoot=\${sourceRoot}/js/src";
 
   preConfigure = ''
+    export CXXFLAGS="-fpermissive"
     export LIBXUL_DIST=$out
   '';